As is known, a high voltage circuit breaker placed upstream on a high voltage electricity line must be capable of interrupting line faults that can happen downstream at distances of as much as several kilometers and that are characterized by very high levels of current. For example, when operating with 145 kV at 60 Hz, the current can be at great at 57 kA.
A problem which arises after an interruption due to a fault of that kind lies in satisfying the conditions laid down at network level for voltage recovery, and in particular the conditions relating to the speed du/dt at which voltage recovers. For example, in the above-envisaged context of 145 kV at 60 Hz, it can be required that voltage recovery takes place at a speed of 13.6 kv/.mu.s.
Since circuit breakers are not capable, at present, of complying with such a constraint, it is conventional to provide a capacitor between ground and the line downstream from a circuit breaker so an to limit the speed at which voltage recovers to a value that is lower than the value which can be withstood by the circuit breaker. Nevertheless, the capacitor provided in such a device can be effective only for reducing a recovery voltage downstream from the circuit breaker, and it has no effect on the upstream or "source-side" voltage.
A dead tank circuit breaker is connected directly to a feed busbar on the upstream side and has no significant capacitance capable of acting thereon, the capacitance of the outlet feeder being small, of the order of 50 pF to 100 pF. It must nevertheless be capable of withstanding the sum of the voltage recoveries both upstream and downstream.
In the above-mentioned example of a network, a usual value for the speed du/dt of upstream voltage recovery is 2 kV/.mu.s to 7.8 kV/.mu.s, with the value for the speed of downstream voltage recovery being 13.6 kV/.mu.s. The high value for the upstream voltage recovery speed, i.e. 7.8 kV/.mu.s, is the consequence of high frequency voltage oscillation due to reflections on the first major discontinuity present in the set of busbars linked to the source.
Even if it is possible to envisage halving the speed of downstream voltage recovery by installing a capacitor of high capacitance, e.g., 6 nF to 12 nF for a 145 kV line, it nevertheless remains that the voltage recovery speed at the terminals of the circuit breaker remains very high and much greater than that which can be accepted by a circuit breaker when breaking.
It is also possible to place a capacitor between a phase and ground on the source side, upstream from a circuit breaker in order to reduce the speed of upstream voltage recovery to a value which is compatible with the limit set, i.e. a capacitor in addition to the above-mentioned capacitor placed on the line side. Nevertheless, that solution is not to be recommended since it can be highly damaging if failure of the capacitor upstream from the circuit breaker gives rise to a ground fault.